Preheat the oven to 350 degrees.
Using a mandolin, slice sweet potatoes into thin slices. (if preferred, peel potatoes before using the mandolin, but not neccesary)
In a medium skillet, heat coconut oil over medium high heat. Add onion and garlic, stir fry for 3-4 minutes.
Add cumin, chili powder, salt and pepper. Stir fry for an additional minute, until fragrant.
Stir in coconut milk and bring to a simmer. In a small bowl, mix tapioca flour with water to create a slurry. Add slurry to the simmering coconut milk and stir to thicken.
Add coconut milk/onion mixture to the base of a 8x12 casserole dish. Arrange sweet potato slices "standing up" in the casserole dish.
Cover with foil and bake for 60 minutes. Remove the foil and cook for another 15 minutes, to brown the top.
Top with fresh parsley and toasted nuts (optional).